Calls enableMustUsePlugin as an instance method

[ffflabs]

Currently, the default value for the enable_must_use_plugin setting is obtained as

'default'      => \Rollbar\Wordpress\Defaults::enableMustUsePlugin(),

rollbar-php-wordpress/src/Settings.php

Lines 206 to 208 in d85766b

	'type' => UI::getSettingType('enable_must_use_plugin'),
	'default' => \Rollbar\Wordpress\Defaults::enableMustUsePlugin(),
	'description' => __('Allows Rollbar plugin to be loaded as early ' .

But enableMustUsePlugin is an instance method, as shown on:

rollbar-php-wordpress/src/Defaults.php

Lines 39 to 43 in d85766b

	public function enableMustUsePlugin()
	{
	return false;
	}

So it throws a deprecation warning:

PHP Deprecated:  Non-static method Rollbar\Wordpress\Defaults::enableMustUsePlugin() 
should not be called statically  in 
/var/www/blog.example.com/wp-content/plugins/rollbar/src/Settings.php on line 207

In this PR I'm fixing said warning by using:

'default'      => \Rollbar\Wordpress\Defaults::instance()->enableMustUsePlugin(),

I hope it helps.

[jessewgibbs]

@amenadiel thanks for the PR.

@ArturMoczulski can you review when you get a chance?

[ArturMoczulski]

@amenadiel great. thanks for catching this. releasing now as 2.5.1